Photoluminescence imaging with preferential detection of photoluminescence signals emitted from a specified material layer of a wafer or other workpiece

Abstract

A method and apparatus uses photoluminescence to identify defects in one or more specified material layers of a sample. One or more filtering elements are used to filter out predetermined wavelengths of return light emitted from a sample. The predetermined wavelengths are selected such that only return light emitted from one or more specified material layers of the sample is detected. Additionally or alternatively, the wavelength of incident light directed into the sample may be selected to penetrate the sample to a given depth, or to excite only one or more selected material layers in the sample. Accordingly, defect data characteristic of primarily only the one or more specified material layers is generated.

What is claimed is: 
     
       1. A method for using photoluminescence, comprising:
 directing a laser at a sample having a first material layer and a second material layer, with the first material layer emitting a first return light and with the second material layer emitting a second return light, wherein the first return light and second return light together form combined return light; 
 filtering the combined return light to pass the first return light and to remove the second return light; 
 detecting the passed first return light; and 
 generating first defect data characteristic of the first material layer, using the detected first return light. 
 
     
     
       2. The method of  claim 1  wherein filtering comprises allowing only return light having a wavelength within a predetermined range to pass through the filter. 
     
     
       3. The method of  claim 1  wherein filtering comprises either:
 (a) allowing only return light having a wavelength greater than a predetermined wavelength to pass through the filter; or 
 (b) allowing only return light having a wavelength less than a predetermined wavelength to pass through the filter. 
 
     
     
       4. The method of  claim 1  wherein the sample includes at least a third material layer, and wherein the laser also penetrates the third material layer. 
     
     
       5. The method of  claim 4  wherein the filtering further comprises filtering out third return light emitted from the third material layer. 
     
     
       6. The method of  claim 4  further comprising detecting the third return light. 
     
     
       7. The method of  claim 4  wherein the first material layer is located between the second and third material layers. 
     
     
       8. The method of  claim 1  further comprising:
 penetrating the first material layer and the second material layer of the sample with an additional laser; 
 detecting additional return light emitted from the first and second material layers to generate second defect data characteristic of defects in the first and second material layers; and 
 subtracting the first defect data from the second defect data to produce third defect data characteristic of defects located primarily in the second material layer. 
 
     
     
       9. The method of  claim 8  further comprising normalizing the first and second defect data relative to each other before performing the subtraction. 
     
     
       10. The method of  claim 1  wherein the first material layer is one of a surface layer and a bulk layer of the sample. 
     
     
       11. The method of  claim 10  wherein the first material layer is a surface layer comprising SiGe, and the second material layer is a bulk region comprising Si. 
     
     
       12. A method for using photoluminescence, comprising:
 directing a laser beam of a selected wavelength toward a sample to penetrate the sample approximately to a desired depth, the sample emitting return light in response to the directed laser beam, the return light including a first return light that is emitted from a first material layer of the sample and a second return light that is emitted from a second material layer of the sample, wherein the desired depth approximately corresponds to the thickness of a first material layer; 
 filtering out a second return light portion of the emitted return light from the sample; and 
 detecting a non-filtered first return light portion of the emitted return light to identify defects in the sample. 
 
     
     
       13. The method of  claim 12  wherein the desired depth is approximately equal to a thickness of a surface layer of the sample. 
     
     
       14. A method for using photoluminescence, comprising:
 selecting a laser beam having a wavelength capable of exciting a first material layer, while not exciting a second material layer, in a sample; 
 directing the laser beam toward the sample; and 
 detecting return light emitted from the excited first material layer to identify defects located in the first material layer. 
 
     
     
       15. The method of  claim 14  wherein an energy of the selected laser beam is greater than an energy band gap of the first material layer, and is less than an energy band gap of the second material layer. 
     
     
       16. A photoluminescence imaging apparatus, comprising:
 a light source for generating a light beam toward a sample; 
 a filter for filtering return light emitted from the sample, wherein the filter allows one or more wavelengths of return light emitted from at least a first material layer of the sample to pass through the filter, while preventing one or more additional wavelengths of return light emitted from at least a second material layer of the sample from passing through the filter; and 
 a detector for detecting the first return light that passes through the filter. 
 
     
     
       17. The apparatus of  claim 16  further comprising a collector for collecting the first and second return light. 
     
     
       18. The apparatus of  claim 17  wherein the filter is positioned between the collector and the detector. 
     
     
       19. A photoluminescence imaging apparatus, comprising:
 means for generating a light beam toward a sample; 
 means for filtering return light emitted from the sample such that first return light emitted from at least a first material layer of the sample is allowed to pass through the filter, while second return light emitted from at least a second material layer of the sample is prevented from passing through the filter; and 
 means for detecting the first return light that passes through the filtering means.